A method and an apparatus for operating an internal combustion engine, in whose exhaust gas region an SCR catalytic converter (Selective-Catalytic-Reduction) is disposed, are described in the German patent DE 199 03 439 A1. Said SCR catalytic converter reduces the nitrogen oxides contained in the exhaust gas of the internal combustion engine to nitrogen with a reagent.
Provision is, for example, made for the reducing agent ammonia to be the reagent. Said reducing agent can be extracted form a urea/water solution, respectively from a urea/formic acid/water solution as a precursor of the reagent. The dosage of the reagent or the precursor of the reagent must be carefully established. Too low of a dosage results in the nitrogen oxide no longer being able to be sufficiently reduced in the SCR catalytic converter. Too high of a dosage leads to a reagent slip, which can lead on the one hand to an unnecessarily high reagent consumption and on the other hand, depending on the properties of the reagent, to an unpleasant odor.
According to the European patent EP 1 024 254 A2, the determination of the reagent flow rate, respectively the metered amount of reagent, can take place based on an operating variable of the internal combustion engine, for example, the amount of fuel injected and/or the engine rotational speed and if need be at least one parameter of the exhaust gas, for example the exhaust gas temperature.
In the German patent DE 10 2006 044 080 A1, a method for operating a metering valve is described, which is implemented as an electromagenetically actuated valve. The metering valve is loaded with a pulse-width modulated metering signal, which defines the metering of a precursor of a reagent, which is to be introduced into the exhaust gas region of an internal combustion engine. The opening duration of the metering valve is limited to a minimum opening duration, for which the metering valve is opened completely and is set in such a way that a spray mist is always produced during the metering. In so doing, the reagent is prevented from crystallizing. The known method firstly ensures exact metering of the reagent and secondly counteracts clogging of the metering valve. The minimum opening duration depends on a plurality of variables, as, for example, the temperature and the operating voltage of the metering valve's electromagnet as well as the reagent pressure.
In the German patent DE 37 10 467 C1, a fuel injection valve is described, which contains a core surrounded by an electromagnet as well as an armature, which interacts with the core, which is fixedly connected to a valve needle. When the electromagnet is energized, the electromagnet attracts the armature and thereby unblocks an opening for the metering of the pressurized fuel up until the electromagnet is de-energized.
The task underlying the invention is to state a method for operating a metering valve, which meters a reagent or a precursor of the reagent into the exhaust gas region of an internal combustion engine, and an apparatus for implementing the method, which makes a metering possible, which is as exact as possible.